Add support to extend the first app's template (empty namespace)

Fixes https://github.com/Fantomas42/django-app-namespace-template-loader/issues/1
Clarify existing test_extend_and_override test

I found it confusing that the used `block` was not used/checked via the
mark, and therefore added explicit asserts for it.
PEP8
Fix import for Python < 2.7 with Django's SortedDict
Only use SortedDict bundled with Django
Set maxDiff to None globally
Reorder mark_title tests
Testing the behavior of load_template_source methode with an empty namespace
Cover the code at 100%
Except assertHTMLEqual under Python > 2.7.3 and Django 1.4
Update the README, close #1
Merge branch 'feature/circular-extends' into develop